The present invention relates to wireless communication systems and, more particularly, to methods and apparatus for communicating control information in wireless communications systems.
Cellular radio systems have been operating in the United States since the early 1980s. In a typical cellular radio system as shown in FIG. 1, a geographical area (e.g., a metropolitan area) is divided into several smaller, contiguous radio coverage areas (called xe2x80x9ccellsxe2x80x9d) such as cells C1-C10. The cells C1-C10 are served by a corresponding group of fixed radio stations (called xe2x80x9cbase stationsxe2x80x9d) B1-B10, each of which operates on a subset of the radio frequency (RF) channels assigned to the system. The RF channels allocated to any given cell may be reallocated to a distant cell in accordance with a frequency reuse pattern as is well known in the art. The cellular telephone users (mobile station subscribers) in the cells C1-C10 are provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) wireless stations, such as mobile stations M1-M9, each of which communicates with a nearby base station. The base stations B1-B10 are connected to and controlled by a mobile station services switching center (MSC) 20. The MSC 20, in turn, is connected to a central office (not shown in FIG. 1) in the landline (wireline) public switched telephone network (PSTN) or to a similar facility such as an integrated system digital network (ISDN). The MSC 20 switches calls between and among wireline and mobile station subscribers, controls signaling to the mobile stations, compiles billing statistics, and provides for the operation, maintenance and testing of the system.
In each cell, at least one RF channel (called the xe2x80x9ccontrolxe2x80x9d or xe2x80x9cpaging/accessxe2x80x9d channel) is used to carry control or supervisory messages, and the other RF channels (e.g., the xe2x80x9cvoicexe2x80x9d or xe2x80x9cspeechxe2x80x9d channels) are used to carry voice conversations. When turned on (powered up), each of the mobile stations M1-M9 enters the idle state (standby mode) and tunes to and continuously monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment).
To detect incoming calls, the mobile station continuously monitors the control channel to determine whether a page message addressed to it (ie., containing its MIN) has been received. A page message will be sent to the mobile station, for example, when an ordinary (landline) subscriber calls the mobile station subscriber. The call is directed from the PSTN to the MSC 20 where the dialed number is analyzed. If the dialed number is validated, the MSC 20 requests some or all of the base stations B1-B10 to page the called mobile station throughout their corresponding cells C1-C10. Each of the base stations B1-B10 which receive the request from the MSC 20 will then transmit over the control channel of the corresponding cell a page message containing the MIN of the called mobile station. Each of the idle mobile stations M1-M9 which is present in that cell will compare the MIN in the page message received over the control channel with the MIN stored in the mobile station. The called mobile station with the matching MIN will automatically transmit a page response over the control channel to the base station which then forwards the page response to the MSC 20. Upon receiving the page response, the MSC 20 selects an available voice channel in the cell from which the page response was received (the MSC 20 maintains an idle channel list for this purpose), and requests the base station in that cell to order the mobile station via the control channel to tune to the selected voice channel. A through-connection is established once the mobile station has tuned to the selected voice channel.
FIG. 3 shows an exemplary DCCH superframe which includes at least three logical channels, namely, a broadcast control channel (BCCH), a paging channel (PCH), and an access response channel (ARCH). The BCCH, which in this example is allocated 6 DCCH slots, carries overhead messages. The PCH, which is allocated one DCCH slot, carries page messages. The ARCH, which is also allocated one DCCH slot, carries voice or speech channel assignment messages. The exemplary superframe of FIG. 3 may contain other logical channels, including additional paging channels (if more than one PCH is defined, different groups of mobile stations may be assigned to different PCHs).
A mobile station operating on the DCCH of FIG. 3 need only be xe2x80x9cawakexe2x80x9d (monitoring) during certain time slots (e.g., the BCCH and its assigned PCH) in each superframe and can enter xe2x80x9csleep modexe2x80x9d at all other times. While in sleep mode, the mobile station turns off most internal circuitry and saves battery power. Furthermore, by configuring the BCCH as taught in U.S. Pat. No. 5,404,355 to Raith, the mobile station can read (i.e., decode) the overhead messages when locking onto the DCC (e.g., at power-up) and thereafter only when the information has changed, thus resulting in additional battery power savings while allowing for fast cell selection.
Typically only a fraction of the page messages received over the PCH will be directed to the mobile station, as most messages will either be empty messages (xe2x80x9cfillerxe2x80x9d messages containing no page) or pages to other mobile stations. The PCH will usually be operated substantially below the capacity limit in order to avoid excessive traffic blocking (and, hence, delay in delivering pages to the mobile stations). If blocking problems do develop (e.g., because of unanticipated demand) in any cell, the operator can assign additional control channels in that cell or use other capacity-enhancing techniques such as cell splitting. Thus, in general, an appropriately-managed PCH typically will be operated at a level far below maximum capacity, even at busy times. Consequently, more often than not, the PCH carries empty messages. Furthermore, since a mobile station usually receives no more than a few calls each day, most of the non-empty page messages sent on the PCH will be for other mobile stations.
To maximize sleep mode efficiency, the mobile station should be able to detect whether the received page messages are relevant messages (e.g., page messages directed to this particular mobile station) or irrelevant messages (e.g., empty page messages or page messages directed to other mobile stations) as early as possible in the receive processing (e.g., after demodulation but before decoding) so as to avoid as many signal processing steps as possible. Once an irrelevant page is detected, the mobile station can immediately return to sleep. To appreciate the possible power savings from an early detection of irrelevant pages, consider a typical PCH in which a page message is sent once per second. This means that there are (60*60*24=) 86,400 page messages sent to the mobile station each day. If, for example, the PCH carries non-empty page messages only 10% of the time, the mobile station can avoid processing 90% of the page messages if it can detect empty pages. Furthermore, if only a few of the non-empty page messages are directed to this mobile station, it can avoid processing almost all of the page messages transmitted on the PCH if it can also detect that the other non-empty page messages are directed to other mobile stations. Thus, the mobile station effectively can be in sleep mode during PCH reception.
The aforementioned U.S. Pat. No. 5,404,355 describes a technique of grouping information elements and providing an indicator whether the mobile station shall read the associated information elements. This technique is used in current IS-136 to inform the mobile stations about new or changed overhead information. In the PCH channel, change flags are provided to indicate the mobile station to read the overhead information and end-user broadcast messages respectively. Within the broadcast channel (BCCH), change flags are provided for to indicate changes, amendments or deletions of the sub-partitioned the BCCH channel.
Similarly, the TIA contribution TR45.5.3.1/98.07.14.13 describes a proposed new channel (F-QPCH) which indicates whether the mobile station shall read it""s assigned sleep mode slot in which there may be pages or overhead information. Each mobile stations is assigned a particular F-QPCH based on it""s identity. Only new mobile stations (designed with the knowledge of the F-QPCH channel) can make benefit of the power savings. Furthermore, in interest of backward compatibility, the F-QPCH channel may have to be introduced on a different control channel then the control channel serving old mobile stations.
U.S. Pat. No. 5,930,706 to Raith describes a technique which can save the mobile station battery power while camped on a control channel. The mobile station re-encodes the format (bit-pattern) of an empty page and compares it with the received page messages before channel decoding. Since there is lots of redundancy in an empty page, the comparison of the stored vector and the received vector can be safely accomplished just using a small fraction of the transmitted data. If the mobile station determines that the received data is an empty page the mobile station stops further processing. In a refined mode of operation, the mobile station re-encodes its paging number (the data used to contact the mobile station e.g. IMSI/MIN/TMSI) and compares the incoming data with the pre-stored data. For example, in the GSM system which interleaves the Paging channel over 4 non-consecutive time slots, the mobile station will most often be able to make a determination whether it shall examine the contents in the Paging channel after having received just one of the four time slots. In TIA/EIA IS-95,the paging slot is rather long (80 ms) and the use of this technique would allow the mobile station to power down, when there is no page present, in a much shorter time. In contrast to the proposal in TR45.5.3.1/98.07.14.13 this technique can be applied to any existing wireless system and does not require any new protocols on the control channel. Furthermore, old and new mobile stations for a given system need not be segregated to different control channels. However, because of possible multiple mobile station identity types and PCH messages that can contain multiple pages the implementation of this method must first be simulated in off-line computer in order for the mobile station not making erroneous decision.
According to embodiments of the present invention, a paging attribute descriptor (PAD) the indicates content of a page message is transmitted in a first time slot and/or a succeeding second time slot of a physical channel, and the page message is transmitted in the second time slot. The second slot may be, for example, a Digital Control Channel (DCCH) slot mapped on a physical channel (DTCH) of an IS-136 system. The transmitted PAD is recovered at wireless station, which then determines whether to recover the page message based on the recovered PAD, e.g., if the PAD indicates that the page message is an empty page message, a page message addressed to another wireless station or a page message that includes control information that has changed. For example, in an IS-136 compliant system, the PAD may be transmitted in the coded superframe phase (CSFP) field of a Paging Channel (PCH) message transmitted in a Digital Control Channel (DCCH) slot, or in reserved bits of a slot preceding a PCH message.
The present invention arises from the realization that power savings in addition to those provided by conventional sleep mode operations and other conventional power-saving techniques may be achieved by using a paging attribute descriptor (PAD) that can be quickly read and used to determine whether to fully process page messages. The PAD may be incorporated within existing control channel structures such that major modification of wireless communications protocols is not required, and such that existing features, such as existing page message control flags, can still be supported. In addition, the PAD feature be implemented such that new generation wireless stations (e.g., cellular telephones) can enjoy the advantages of using the PAD feature while compatibility with older terminal designs is maintained. In embodiments described herein, this may be achieved by mapping a PAD channel onto existing fields in Paging Channel (PCH) messages and/or Digital Control Channel (DCCH) messages, and by structuring the PAD to support page message control flags and modified link quality monitoring techniques.
According to an embodiment of the present invention, in a wireless communications system that is operative to communicate over a physical channel defined as a series of repeating time slots, a paging attribute descriptor (PAD) is transmitted in at least one of a first time slot of the physical channel and a second time slot of the physical channel succeeding the first time slot, the PAD indicating content of a page message. The page message is transmitted in the second time slot. The PAD at a wireless station, which determining whether to recover the page message based on the recovered PAD. Preferably, the wireless station recovers the page message if the recovered PAD meets a predetermined criterion and foregoes recovery of the page message if the recovered PAD fails to meet the predetermined criterion.
In one embodiment of the present invention, a signal including the PAD is transmitted in a time slot assigned to a paging channel. The signal is received at the wireless station, and demodulated to an extent sufficient to recover the PAD. The wireless station foregoes further demodulation of the received signal if the recovered PAD meets a predetermined criterion.
In another embodiment, a signal burst representing the page message and the PAD is transmitted in a Digital Control Channel (DCCH) time slot assigned to a Paging Channel (PCH). The signal burst is received at the wireless station, and processed to recover the PAD. The signal burst may represent a physical layer message including a Coded Super Frame Phase (CSFP) field including the PAD. In another embodiment, a similar technique is used to transmit a PAD in a Packet data Control Channel (PCCH) time slot assigned to a Paging Channel (PCH), more particularly, in a Coded Data Frame Type (CDFT) field.
In yet another embodiment of the present invention, a first signal burst representing the PAD is transmitted in a time slot preceding a DCCH time slot assigned to a Paging Channel (PCH). A second signal burst representing the page message is transmitted in the DCCH time slot assigned to the PCH. The second signal burst may be sufficiently processed at the wireless station to recover synchronization information, which is then used to process the first signal burst to recover the PAD. The wireless station may then determine whether to farther process the second signal burst to recover the page message based on the recovered PAD. According to yet another aspect of the present invention, a transmitted PAD includes one of a plurality of group values, a respective one of the group values associated with a respective group of wireless stations. This group value may be recovered at a receiving wireless station, which recovers the page message if the recovered group value is associated with a group of wireless stations of which the wireless station is a member.
In another embodiment of the present invention, a transmitted PAD includes a flag indicating status of control information included in a page message. This flag may be recovered at the wireless station, which determines whether to recover the associated page message based on the recovered flag.
According to still another aspect of the present invention, a decision whether to recover a page message at a wireless station based on a recovered PAD is biased towards one of recovering the page message or foregoing recovery of the page message. For example, the wireless station may bias the decision whether to recover a page message based on a recovered PAD associated with the page message more towards recovering the page message than foregoing recovery of the page message, as missing a page message that is actually intended for the wireless station may have more significant negative consequences than unnecessarily reading a page. The biasing may be based, for example, on channel quality.
In still another embodiment of the present invention, a wireless station may operate in first and second modes in which it process PADs differently. The wireless station may recover a transmitted page message at the wireless station if the associated PAD meets a predetermined criterion and the wireless station is in the first mode. However, if the wireless station is in the second mode, it may recover the transmitted page message irrespective of the recovered PAD.
According to another aspect of the present invention, a wireless station includes a receiver that recovers a transmitted paging attribute descriptor (PAD) from at least one of a first time slot of a physical channel defined as a series of repeating time slots and a second time slot of the physical channel succeeding the first time slot, the PAD indicating content of a page message transmitted in the second time slot, and that determines whether to recover the page message based on the recovered PAD. The receiver may be operative to recover the page message if the recovered PAD meets a predetermined criterion and to forego recovery of the page message if the recovered PAD fails to meet the predetermined criterion.
According to yet another aspect of the present invention, a wireless communications system includes a base station that transmits a paging attribute descriptor (PAD) in at least one of a first time slot of a physical channel comprising a series of repeating time slots and a second time slot of the physical channel succeeding the first time slot, the PAD indicating content of a page message, and that transmits the page message in the second time slot. The PAD may be transmitted, for example, in a CSFP field of a PCH page message transmitted in a DCCH slot and/or in a slot preceding this DCCH slot.